Nickel sulphide (NiS) stones are sometimes found as impurities in glass. It is known that such nickel sulphide stones cause spontaneous fracture in toughened window glass. The spontaneous fracture can cause serious problems in high-rise office buildings, where large areas of glass are used.
It is very difficult to detect the presence of NiS stones at the time of glass manufacture. The glass scanners on the glass production line cannot detect stones smaller than 1 mm in size. NiS is not a common impurity in glass and when it does occur, the stones range in size from 0.1 to 0.6 mm. It is believed that any nickel sulphide stone of diameter greater than 70 .mu.m is capable of breaking toughened glass, but stones smaller than 70 .mu.m do not normally break the glass.
Most batches of glass have very few nickel sulphide stones, but occasionally bad batches are produced. Problems resulting from the nickel sulphide impurities often do not become apparent until that glass is placed onto a building many months after manufacture. Consequently, it is very difficult for glass manufacturers to track down what actually caused the high incidence of NiS in that particular batch.
The applicant's earlier international patent application no. PCT/AU93/00498 described a method of detection of defects in glass under controlled lighting conditions. (The disclosure of that patent application is incorporated herein by reference). Although the method described in international patent application no. PCT/AU93/00498 was able to locate the majority of nickel sulphide stones in window glass, the method had a number of inherent disadvantages.
First, the method was conducted entirely on site, and hence was dependent on weather conditions. Secondly, the method was tedious and time consuming. Thirdly, the method did not located all problematic NiS stones, possibly due to lack of concentration by the person examining the windows as a result of tedium and uncomfortable environment.
It is an object of the present invention to provide an improved method and apparatus for detecting defects in window glass.